The present invention relates to a crystal growing technique and processing technique for easily obtaining semiconductor lasers which have a desired structure for confining optical transverse modes.
Namely, the present invention relates to a process for fabricating semiconductor lasers of the internal stripe self-aligned type formed by the organometal chemical vapor deposition method or by the molecular beam epitaxial method, and more specifically to a method which prevents the formation of a degradation layer in the step of buried epitaxy.
To easily control the transverse mode of GaAlAs visible semiconductor lasers, there has been known a structure according to which an n-type GaAs layer is put on a p-type GaAlAs cladding layer, GaAs is removed from a laser active region only, and a p-type GaAlAs cladding layer is grown thereon.
According to this method, however, the surface of the active GaAlAs layer is exposed to the air; i.e., the surface is easily oxidized and defects develop in the interface when the cladding layer is formed thereon.
On the other hand, the GaAs layer is oxidized little and a good interface is obtained if the cladding layer is formed thereon. According to Tanaka et al., Applied Physics, Vol. 54, No. 11, 1985, p. 1209, Section 3, "Method of Producing Self-Aligned Lasers", a crystal is introduced into a molecular beam epitaxy (MBE) apparatus with a GaAs layer having a thickness of 0.1 to 0.2 .mu.m being left on a p-GaAlAs layer, and is heated while it is being irradiated with an As molecular beam, and a thin GaAs layer is thermally etched so that a cladding p-GaAlAs layer is exposed under a high vacuum condition, and a p-GaAlAs layer is grown thereon. This method, however, requires an MBE apparatus which must maintain an ultra-high vacuum condition, and is not usable in the OMVPE (organometallic vapor phase epitaxy) that is tailored to mass-produce the devices.
In order to easily control the transverse mode in GaAlAs visible semiconductor lasers, there has been known a structure (SAS) in which an n-type GaAs layer is formed on a p-type GaAlAs cladding layer, the n-type GaAs layer is removed from the corresponding region of the laser active region only, and a p-type GaAlAs cladding layer is grown again (see the literature described above).
According to this method, however, the surface of the active GaAlAs cladding layer is exposed to the air. Therefore, the surface is oxidized and defects develop when the crystal is grown thereafter.
On the other hand, the GaAs layer is oxidized little, and a good interface is obtained if the crystal is grown thereon. According to Tanaka et al. (see the literature described above), a crystal is introduced into a molecular beam epitaxy (MBE) apparatus with a GaAs layer having a thickness of 0.1 to 0.2 .mu.m being left on a p-GaAlAs layer, and is heated while it is being irradiated with an As molecular beam, and a thin GaAs layer is thermally etched so that a cladding p-GaAlAs layer is exposed under a high vacuum condition, and a p-GaAlAs layer is grown again thereon. This method, however, requires an ultra-high vacuum condition and is not usable for the MOCVD (metal organic chemical vapor deposition) that is tailored to mass-produce the devices.